The rarest type of twins is sesquizygotic, or “semi-identical.” Only two confirmed cases exist worldwide: one identified in the United States in 2007 and another in Queensland, Australia, identified during pregnancy in 2019. These twins fall between identical and fraternal on the genetic spectrum, sharing all of their mother’s DNA but only a portion of their father’s.
But semi-identical twins aren’t the only extraordinary rarity. Several other types of twinning are so uncommon that most obstetricians will never encounter them in an entire career. Here’s how the rarest forms compare.
Semi-Identical (Sesquizygotic) Twins
In a normal conception, one sperm fertilizes one egg. In sesquizygotic twinning, two sperm fertilize the same egg simultaneously. That creates an embryo with three sets of chromosomes: one from the mother and one from each of the two sperm. Three sets of chromosomes aren’t viable on their own, so the embryo does something remarkable. It sorts those chromosomes into two groups of cells, each with the normal two sets, and then splits into two embryos.
The result is a pair of twins who are identical on their mother’s side but share only some of their father’s DNA. The Queensland twins, a boy and a girl, were first noticed during a routine ultrasound when doctors saw they shared a single placenta (typical of identical twins) yet were different sexes (typical of fraternal twins). Genetic testing after birth confirmed the sesquizygotic mechanism. With just two documented cases in all of medical literature, this is the rarest confirmed form of human twinning.
Twins With Different Fathers
Heteropaternal superfecundation happens when a woman releases two eggs in the same cycle, has sex with two different partners, and each egg is fertilized by a different man’s sperm. The twins are fraternal but share only half the genetic relationship that fraternal twins normally do, making them more like half-siblings born at the same time.
As of 2020, only 19 cases had been reported worldwide. The true number is almost certainly higher, since these twins look like ordinary fraternal twins and are only discovered when someone questions paternity and orders a DNA test. The biological prerequisite is hyperovulation, where the ovaries release more than one egg in a single menstrual cycle. Women with naturally high levels of the hormone that triggers ovulation are more likely to release multiple eggs, but the two-partner element makes this vanishingly rare in practice.
Superfetation: Conceiving While Already Pregnant
Superfetation occurs when a woman conceives a second baby days or even weeks after the first. This requires ovulation, fertilization, and implantation to all happen while a pregnancy is already underway, something the body’s hormonal shifts are specifically designed to prevent. Fewer than ten cases have ever been reported in humans, and according to Duke University’s obstetrics department, it’s not clear any have been definitively proven. The babies are born at the same time but may differ slightly in gestational age, which is what tips off doctors during ultrasound measurements.
Conjoined Twins
Conjoined twins form when a single fertilized egg begins to split into identical twins but stops before the process is complete. The result is two babies who are physically connected, sharing skin, organs, or both depending on where the split stalled. A large epidemiological study coordinated through the International Clearinghouse for Birth Defects Surveillance and Research found a prevalence of about 1.47 per 100,000 births, consistent with older estimates of roughly 1 in 50,000 to 1 in 100,000.
Survival is a major concern. Fetal anomalies are common, and the complexity of shared anatomy means surgical separation is only possible in some cases. The specific type of connection varies widely. Some pairs share only a small bridge of tissue, while others share a heart or liver.
Monoamniotic (MoMo) Twins
Monoamniotic twins, often called MoMo twins, are identical twins who share both a placenta and a single amniotic sac. Most identical twins share a placenta but develop in separate sacs with a thin membrane between them. When the fertilized egg splits late enough, that dividing membrane never forms, and the two babies float freely in the same fluid-filled space.
This creates a dangerous situation. The umbilical cords can tangle and compress, cutting off blood flow. Fetal anomalies occur in 15 to 25 percent of monoamniotic pregnancies, and conditions like twin-to-twin transfusion syndrome, where blood flow between the babies becomes unbalanced, add further risk. Overall, only about 70 percent of monoamniotic twins survive. Those who do are often born prematurely, and the complications of early delivery drive much of the health challenges after birth. Most MoMo pregnancies are monitored intensively, often with extended hospital stays in the third trimester so doctors can track both heartbeats continuously.
Chimerism: When Twin Embryos Merge
Tetragametic chimerism is in some ways the opposite of twinning. Two separate eggs are fertilized by two separate sperm, just as in a normal fraternal twin pregnancy. But instead of developing into two babies, the embryos fuse very early in development, producing a single person whose body contains two distinct sets of DNA. Some organs or tissues carry one genetic blueprint while others carry the second.
This is extraordinarily rare and usually discovered by accident, often during organ transplant matching, forensic investigations, or paternity tests that return seemingly impossible results. In rare situations, one of the original embryos was genetically male and the other female, which can lead to a single individual with both XX and XY cells in different parts of their body. The chances are slightly elevated with IVF, since multiple embryos are implanted in close proximity, increasing the opportunity for fusion.
Mirror Image Twins
Mirror image twins are identical twins whose features are reversed, as if one twin is the other’s reflection. One might be left-handed while the other is right-handed, or a birthmark on one twin’s left cheek appears on the other’s right. About 25 percent of identical twins show this pattern. It’s thought to result from a later-than-usual egg split, around 7 to 10 days after conception instead of the more typical split within the first week. Mirror twins aren’t medically risky and aren’t classified as a separate type in the way conjoined or monoamniotic twins are, but they’re a well-documented curiosity within identical twinning.
Why Multiple Births Are Shifting
Twinning rates climbed sharply in the 1980s and 1990s, driven by the rise of fertility treatments that used hormone stimulation and the implantation of multiple embryos. Since about 2000, that trend has reversed. Modern IVF protocols typically transfer a single embryo, because success rates have improved enough that implanting two or three is no longer necessary. Awareness of the complications that come with multiple pregnancies has also pushed clinics toward single-embryo transfer. In the Netherlands, for example, multiple births made up 1.51 percent of all live births in 2024, a figure that has held roughly steady over the past decade after the earlier decline.
Maternal age still plays a role. Older mothers are more likely to release multiple eggs in a single cycle, which increases the chance of fraternal twins. But the overall trend in assisted reproduction has been toward fewer multiples, not more.